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      MySQL学习笔记_day1
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    <h2 id="1-数据库的好处"><a href="#1-数据库的好处" class="headerlink" title="1 数据库的好处"></a>1 数据库的好处</h2><div class="post-note note-warning"><p>1、持久化数据到本地<br>2、可以实现结构化查询，方便管理</p></div>

<h2 id="2-数据库相关概念"><a href="#2-数据库相关概念" class="headerlink" title="2 数据库相关概念"></a>2 数据库相关概念</h2><div class="post-note note-warning"><p>1、DB：数据库，保存一组有组织的数据容器<br>2、DBMS：数据库管理系统，又称为数据库软件（产品），用于管理DB中的数据<br>3、SQL：结构化查询语言，用于和DBMS通信的语言</p></div>

<h2 id="3-数据库存储数据的特点"><a href="#3-数据库存储数据的特点" class="headerlink" title="3 数据库存储数据的特点"></a>3 数据库存储数据的特点</h2><div class="post-note note-warning"><p>1、将数据放到表中，表再放到库中<br>2、一个数据库中可以有多个表，每个表都有一个名字，用来标识自己，表名具有唯一性<br>3、表具有一些，这些特性定义了数据在表中如何存储，类似“类”的设计<br>4、表由列组成，我们也称为字段，所有表都是由一个或者多个列组成的，每一列类似于“类”中的属性</p></div>

<h2 id="4-MySQL的安装和介绍"><a href="#4-MySQL的安装和介绍" class="headerlink" title="4 MySQL的安装和介绍"></a>4 MySQL的安装和介绍</h2><h3 id="4-1-MySQL的安装"><a href="#4-1-MySQL的安装" class="headerlink" title="4.1 MySQL的安装"></a>4.1 MySQL的安装</h3><p>原文安装教程：<a target="_blank" rel="noopener" href="https://hxf233333.github.io/2021/02/21/%E5%AE%89%E8%A3%85MySQL/">安装MySQL</a></p>
<h3 id="4-2-MySQL服务的启动和停止"><a href="#4-2-MySQL服务的启动和停止" class="headerlink" title="4.2 MySQL服务的启动和停止"></a>4.2 MySQL服务的启动和停止</h3><div class="post-note note-warning"><p>方式一：计算机-&gt;右击管理-&gt;服务<br>方式二：通过管理员身份运行<br>        <code>net start mysql</code> （启动mysql服务）<br>        <code>net stop mysql</code>（停止mysql服务）</p></div>

<h3 id="4-3-MySQL服务的登录和退出"><a href="#4-3-MySQL服务的登录和退出" class="headerlink" title="4.3 MySQL服务的登录和退出"></a>4.3 MySQL服务的登录和退出</h3><div class="post-note note-warning"><p>方式一：通过mysql自带的客户端（只限于root用户）</p><p>方式二：通过windows自带的DOS命令<br>        登录：<code>mysql 【-h主机名 -P端口号】 -u用户名 -p密码</code><br>        退出：<code>exit; 或 Ctrl + C</code></p></div>

<h3 id="4-4-MySQL的常见命令"><a href="#4-4-MySQL的常见命令" class="headerlink" title="4.4 MySQL的常见命令"></a>4.4 MySQL的常见命令</h3><div class="post-note note-warning"><p>1、查看当前所有的数据库</p><pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">show</span> <span class="token keyword">databases</span>；<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre><p>2、打开指定的库</p><pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">use</span> 库名；<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre><p>3、查看当前库的所有表</p><pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">show</span> <span class="token keyword">tables</span>；<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre><p>4、查看其他库的所有表</p><pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">show</span> <span class="token keyword">tables</span> <span class="token keyword">from</span> 库名；<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre><p>5、创建表</p><pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">create</span> <span class="token keyword">table</span> 表名<span class="token punctuation">(</span>
	列名 列类型，
	列名 列类型
	<span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>
	<span class="token punctuation">)</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre><p>6、查看表结构</p><pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">desc</span> 表名；<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre><p>7、查看服务器的版本<br>方式一：登录到mysql服务端</p><pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">select</span> version<span class="token punctuation">(</span><span class="token punctuation">)</span>；<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre><p>方式二：没有登录到mysql服务端<br><code>mysql  --version 或者 mysql --V</code></p></div>

<h3 id="4-5-MySQL的语法规范"><a href="#4-5-MySQL的语法规范" class="headerlink" title="4.5 MySQL的语法规范"></a>4.5 MySQL的语法规范</h3><div class="post-note note-warning"><p>1、不区分大小写，但建议关键字大写，表名、列名小写<br>2、每条命令最好用分号结尾<br>3、每条命令根据需要，可以进行缩进或者换行<br>4、注释<br>    单行注释：#注释文字<br>    单行注释：– 注释文字<br>    多行注释：/*  注释文字   */</p></div>

<h2 id="5-DQL语言"><a href="#5-DQL语言" class="headerlink" title="5 DQL语言"></a>5 DQL语言</h2><p>DQL:Data Query Language  数据查询语言</p>
<h3 id="5-1-基础查询"><a href="#5-1-基础查询" class="headerlink" title="5.1 基础查询"></a>5.1 基础查询</h3><pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token comment">#进阶1:基础查询</span>
<span class="token comment">/*
语法：
select 查询列表 from 表名；
特点：
1、查询列表可以是：表中的字段、常量值、表达式、函数
2、查询的结果是一个虚拟的表格
*/</span>

<span class="token comment">#选择使用的库</span>
<span class="token keyword">USE</span> myemployees<span class="token punctuation">;</span>

<span class="token comment">#1.查询表中的单个字段</span>
<span class="token keyword">SELECT</span> last_name <span class="token keyword">FROM</span> employees<span class="token punctuation">;</span>

<span class="token comment">#2.查询表中的多个字段</span>
<span class="token keyword">SELECT</span> last_name<span class="token punctuation">,</span>salary<span class="token punctuation">,</span>email <span class="token keyword">FROM</span> employees<span class="token punctuation">;</span>

<span class="token comment">#3.查询表中的所有字段</span>
<span class="token keyword">SELECT</span> 
	<span class="token punctuation">`</span>employee_id<span class="token punctuation">`</span><span class="token punctuation">,</span>
	<span class="token punctuation">`</span>first_name<span class="token punctuation">`</span><span class="token punctuation">,</span>
	<span class="token punctuation">`</span>last_name<span class="token punctuation">`</span><span class="token punctuation">,</span>
	<span class="token punctuation">`</span>email<span class="token punctuation">`</span><span class="token punctuation">,</span>
	<span class="token punctuation">`</span>phone_number<span class="token punctuation">`</span><span class="token punctuation">,</span>
	<span class="token punctuation">`</span>job_id<span class="token punctuation">`</span><span class="token punctuation">,</span>
	<span class="token punctuation">`</span>salary<span class="token punctuation">`</span><span class="token punctuation">,</span>
	<span class="token punctuation">`</span>commission_pct<span class="token punctuation">`</span><span class="token punctuation">,</span>
	<span class="token punctuation">`</span>manager_id<span class="token punctuation">`</span><span class="token punctuation">,</span>
	<span class="token punctuation">`</span>department_id<span class="token punctuation">`</span><span class="token punctuation">,</span>
	<span class="token punctuation">`</span>hiredate<span class="token punctuation">`</span>
<span class="token keyword">FROM</span>
	employees<span class="token punctuation">;</span> <span class="token comment">#方式一，依次枚举，''为着重号用于区分关键字</span>

<span class="token keyword">SELECT</span> <span class="token operator">*</span> <span class="token keyword">FROM</span> employees<span class="token punctuation">;</span> <span class="token comment">#方式二，使用通配符</span>
	
<span class="token comment">#4.查询常量值</span>
<span class="token keyword">SELECT</span> <span class="token number">100</span>；
<span class="token keyword">SELECT</span> <span class="token string">'john'</span><span class="token punctuation">;</span>

<span class="token comment">#5.查询表达式</span>
<span class="token keyword">SELECT</span> <span class="token number">100</span><span class="token operator">%</span><span class="token number">98</span><span class="token punctuation">;</span>

<span class="token comment">#6.查询函数</span>
<span class="token keyword">SELECT</span> VERSION<span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	
<span class="token comment">#7.起别名</span>
<span class="token comment">/*
①便于理解
②如果要查询的字段有重名的情况，使用别名区分
*/</span>
<span class="token comment">#方式一：使用AS</span>
<span class="token keyword">SELECT</span> last_name <span class="token keyword">AS</span> 姓<span class="token punctuation">,</span>first_name <span class="token keyword">AS</span> 名 <span class="token keyword">FROM</span> employees<span class="token punctuation">;</span>
<span class="token comment">#方式二：使用空格</span>
<span class="token keyword">SELECT</span> last_name 姓<span class="token punctuation">,</span>first_name 名 <span class="token keyword">FROM</span> employees<span class="token punctuation">;</span>
<span class="token comment">#案例：查询salary，显示结果为out put</span>
<span class="token keyword">SELECT</span> salary <span class="token keyword">AS</span> <span class="token string">"out put"</span> <span class="token keyword">FROM</span> employees<span class="token punctuation">;</span>

<span class="token comment">#8.去重</span>
<span class="token comment">#案例 ：查询员工表中涉及到的所有部门编号</span>
<span class="token keyword">SELECT</span> <span class="token keyword">DISTINCT</span> department_id <span class="token keyword">FROM</span> employees<span class="token punctuation">;</span>
	
<span class="token comment">#9.+号的作用</span>
<span class="token comment">/* 
mysql中的+号只有运算符的功能
select 100+100; 两个操作数都为整型，则做加法运算
select '123'+90; 只要其中一方为字符型，试图将字符型数值转换成数值型
		  如果转转成功，则继续做加法运算
select 'john'+90; 如果转换失败，则将字符型数值转换成0
select null+10；  只要其中一方为null，则结果肯定为null
*/</span>
<span class="token comment">#案例：查询员工名和姓连接成一个字段，并显示为 姓名</span>
<span class="token keyword">SELECT</span> CONCAT<span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">,</span><span class="token string">'b'</span><span class="token punctuation">,</span><span class="token string">'c'</span><span class="token punctuation">)</span> <span class="token keyword">AS</span> 结果<span class="token punctuation">;</span>
<span class="token keyword">SELECT</span>
	CONCAT<span class="token punctuation">(</span>last_name<span class="token punctuation">,</span><span class="token string">' '</span><span class="token punctuation">,</span>first_name<span class="token punctuation">)</span> <span class="token keyword">AS</span> 姓名
<span class="token keyword">FROM</span>
	employees<span class="token punctuation">;</span>
	
<span class="token comment">#10.补充函数</span>
<span class="token keyword">SElECT</span> IFNULL<span class="token punctuation">(</span>字段名<span class="token punctuation">,</span>表达式<span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
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    <p><span style="font-weight: bold;">作者：</span><a target="_blank" rel="nofollow noopener noreferrer" href="https://hxf233333.gitee.io/about">hxf233333</a></p>
    <p><span style="font-weight: bold;">文章链接：</span><a target="_blank" rel="nofollow noopener noreferrer" href="https://hxf233333.gitee.io/2021/02/24/MySQL%E5%AD%A6%E4%B9%A0%E7%AC%94%E8%AE%B0_day1/">https://hxf233333.gitee.io/2021/02/24/MySQL%E5%AD%A6%E4%B9%A0%E7%AC%94%E8%AE%B0_day1/</a></p>
    <p><span style="font-weight: bold;">版权声明：</span>本博客所有文章除特别声明外，均采用<a target="_blank" rel="noopener" href="https://creativecommons.org/licenses/by-nc-sa/4.0/deed.zh">CC BY-NC-SA 4.0 协议</a>。转载请注明出处！</p>
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